tadpole

Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

n. The limbless aquatic larva of a frog or toad, having gills and a long flat tail. As the tadpole approaches the adult stage, legs and lungs develop, and the tail gradually disappears. Also called polliwog.

from Wiktionary, Creative Commons Attribution/Share-Alike License

n. A young toad or frog in its larval stage of development that lives in water, has a tail and no legs, and, like a fish, breathes through gills.

from the GNU version of the Collaborative International Dictionary of English

n. The young aquatic larva of any amphibian. In this stage it breathes by means of external or internal gills, is at first destitute of legs, and has a finlike tail. Called also polliwig, polliwog, porwiggle, or purwiggy.

n. The hooded merganser.

from The Century Dictionary and Cyclopedia

n. The larva of a batrachian, as a frog or toad, from the time it leaves the egg until it loses its gills and tail.

n. The hooded merganser, Lophodytes cucullatus: doubtless so called from the apparent size of the head. See the quotation under moss-head.

It is usually thought to be a clipping of tadpole, though it could also be a dialectal variant of toad (from which the first element in tadpole is itself derived), which has also been used as a humorous term of address for a small boy.